F5 WAF Stable Software Version Recommendation

if you hit a specific issue in v16.1 you should really raise a Support issue with F5 - this way all F5 customers can benefit from your finding being addressed.

Personally I found v13.1.x branch to be pretty good for WAF and many of my customers are still using it (it only makes sense if you don't need the new WAF features brought in by v15 and v16), however be aware that v13.1.x support will be discontinued next year in 2023) - do check the K5903: BIG-IP software support policy to see the end of support dates:

https://support.f5.com/csp/article/K5903